In a typical cellular communications network, also referred to as a wireless communication system, user equipments communicate via a Radio Access Network (RAN) to one or more core networks.
A user equipment is a mobile terminal by which a subscriber can access services offered by an operator's core network. The user equipments may be for example communication devices such as mobile telephones, cellular telephones, laptops or tablet computers, sometimes referred to as surf plates, with wireless capability. The user equipments may be portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another mobile station or a server.
User equipments are enabled to communicate wirelessly in the cellular communications network. The communication may be performed e.g. between two user equipments, between a user equipment and a regular telephone and/or between the user equipment and a server via the radio access network and possibly one or more core networks, comprised within the cellular network.
The cellular communications network covers a geographical area which is divided into cell areas, where radio coverage is provided by a base station at a base station site. wherein each cell area being served by a base station, A base station may be e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “NodeB”, “B node”, or Base Transceiver Station (BTS), depending on the technology and terminology used. In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations may be referred to as eNodeBs or eN Bs. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations.
The base stations may be of different classes such as e.g. macro base stations, macro eNodeBs, home eNodeBs or pico base stations, based on transmission power and thereby also cell size.
In some radio access networks, several base stations may be connected, e.g. by landlines or microwave. Interfaces between base stations specifying how the base stations may communicate with each other using the connections have been specified, e.g. by 3GPP.
A heterogeneous network may be characterized as deployments with a mixture of cells of differently sized and overlapping coverage areas. Heterogeneous networks have recently attained large interest within the mobile cellular industry and are regarded by operators and many vendors as the deployment needed to meet high user experiences in mobile broadband. One example of such network is where pico cells are deployed within the coverage area of a macro cell. A pico cell is served by a small cellular base station transmitting with low output power and the pico cell typically covers a much smaller geographical area than a macro cell served by a macro base station. Note that the pico cells may also be placed on different floors in a high rise building. Another name for a pico cell is a Low Power Node (LPN).
Heterogeneous networks represent an alternative to densification of networks of macro cells, and have classically been considered in cellular networks with traffic hotspots as a deployment for increasing network capacity. In emerging mobile broadband applications, there is however a continuous demand for higher data rates and therefore it is of interest to deploy low power nodes not necessarily to cover traffic hotspots only but also at locations within the macro cell coverage where the signal-to-noise ratio prevents high data rates.
3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. All data transmission in LTE is controlled by the radio base station.
In the context of this disclosure the expression Down Link (DL) will be used for the transmission path from the base station to the user equipment. The expression UpLink (UL) will be used for the transmission path in the opposite direction i.e. from the user equipment to the base station. In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of 1 ms length.